The three-dimensional atom probe (3DAP), also known as a position-sensitive atom probe (POSAP), is a device which allows specimens to be analyzed on an atomic level. Typical atom probes operate by (usually positively) ionizing and extracting atoms from a specimen's surface. FIG. 1 presents a schematic view of an exemplary atom probe of a more recent type, wherein the locations and identities of the component atoms of a specimen 10 are determined by situating the specimen 10 opposite a position-sensitive detector 100 (generally a microchannel plate and delay line anode). A local electrode 102 is then situated between the specimen 10 and detector 100. The specimen 10 is generally charged to some datum potential Vs (generally a positive voltage between 500 and 20,000 V), and the local electrode 102 is held to some attractive potential Vle. Where the datum potential Vs is positive, the local electrode 102 is often set to ground (0 V). The detector 100 is also charged to a potential Vd which is attractive with respect to Vs. As a result, the atoms of the specimen 10 are attracted towards the local electrode 102 and detector 100 in accordance with their proximity from the local electrode 100 and detector 100 (i.e., atoms of the specimen 10 which are closer to the local electrode 100 are more strongly attracted). However, the magnitude of Vs−Vle—i.e., the attractive force exerted between the local electrode 102 and the specimen 10—is held at some fraction of the value necessary to ionize atoms of the specimen 10.
When it is then desired to ionize atoms, an additional attractive potential—an “over-voltage” Vo—is then momentarily applied to the local electrode 102, usually in brief pulses, so that the total applied potential Vs−(Vle+Vo) will induce atoms to “evaporate” from the specimen 10, ideally with a single atom (ion) of the specimen 10 leaving the specimen 10 each time an over-voltage pulse is applied. Additionally or alternatively, momentary heating of the specimen 10 (as with a laser) can be used to induce ion evaporation. The evaporated ions are accelerated towards the local electrode 102 to pass through an aperture defined therein, and then impinge upon the detector 100. Under ordinary test conditions, the evaporated ions from the specimen 10 project onto the detector 100 at positions correlated with their original locations on the specimen 10, and thus the detector 100 provides data regarding the original position of the ions on the specimen 10. Additionally, ion times of flight (as measured between application of the over-voltage pulse Vo and detector impingement) provide information regarding ion masses, and thus their identities. Thus, repeated overvoltage pulsing allows a three-dimensional map of the locations and identities of the atoms of the specimen 10 to be constructed. Further general information may be found, for example, in U.S. Pat. No. 5,440,124; U.S. Pat. No. 5,061,850; International Publication WO 99/14793; and Kelly et al., Ultramicroscopy 62:29–42 (1996).
One performance limitation of atom probes is their ability to distinguish between ions of nearly similar masses. This property, known as mass resolution, limits the ability to accurately identify ions from the specimen, and leads to uncertainty or errors in the compositional analysis provided by atom probes. Mass resolution limitations are a consequence of the probabilistic nature of ionization: the precise moment at which ionization occurs during an overvoltage pulse Vo can vary slightly between pulses, and thus there are limitations in precisely determining the time of evaporation. Additionally, owing to practical limitations and expense, pulsing systems for applying the overvoltage pulse Vo tend to apply a pulse wherein the amplitude of Vo is not constant over the duration of the pulse, and thus the exact escape potential (and thus velocity and time of flight) of an evaporated ion will vary. While these limitations are expected to diminish as the available pulsing electronics grow in quality (and decrease in expense), they nonetheless pose difficulties given the state of the art as of the year 2003. Several strategies have been employed in atom probes to increase mass resolution, and one strategy (noted in some of the foregoing references) is to situate an intermediate electrode 104 closely adjacent to the local electrode 102, and between the local electrode 102 and the detector 100, and to hold it at some constant attractive potential Vi whereby the velocity of evaporated ions will (at least to some extent) be decoupled from the overvoltage pulse Vo used to induce evaporation. One strategy is to hold Vi at the same potential as Vle, thereby decelerating ions by the amount equivalent to Vo from which the velocity variation originates. Alternatively, an accelerating (more attractive) potential can be applied to Vi, increasing the overall velocity of the ions so that the variation in Vo becomes relatively smaller.
Another limitation of atom probes relates to the time and expense of testing, particularly in the time and expense of preparing specimens for analysis. Initially, the specimens being analyzed must generally be carefully prepared by removing portions of the specimen around the area of interest for study, so that the area of interest is at the tip of a needle-shaped specimen (typically less than 100 nm in diameter). The needle shape creates the large electric field conducive for ionization, allowing the atom probe to operate over a more convenient voltage range (and/or allowing use of less complex thermal pulsing systems). Additionally, since the ions from the needle project onto the detector (and generally “spread” onto the detector in accordance with their relative positions along the axis of the specimen and local electrode), the needle shape assists in attaining atomic-scale resolution in position data for detected ions. However, preparation of specimens—forming their areas of interest at the tips of needles—can be time-consuming and expensive, and can also be difficult where specimens are brittle or otherwise difficult to shape (as is often the case with semiconductor wafer-derived specimens).
Additionally, since the atom probe must be located in a vacuum chamber (and the specimen must be cryogenically cooled) for optimal operation, specimen processing can be slowed by the need to load and purge the chamber, and to achieve the desired degree of specimen cooling, before testing each specimen. The “warm-up” time between specimens can be reduced by situating multiple specimens within the chamber at the same time (or by forming multiple microtips in a specimen), and then laterally repositioning the specimen with respect to the local electrode (or vice versa) so that several microtips can be analyzed in sequence without the need for intermediate load/purge/cool steps. However, there is still room for improvement in the speed of specimen throughput.
To compound the foregoing problems, specimens are often tested only to find that the collected data is incomplete, e.g., the data does not fully represent the regions of the specimen of particular interest. As an example, the atom probe may not have been run for long enough that data is collected from the desired depth within the specimen. Data may also be incomplete because a desired feature may rest partially or wholly outside of the field of view of the atom probe, because the ions from the feature had flight paths which did not impinge upon the detector. It is also possible that data from the desired feature may not be collected at the desired magnification: the detector has a limit to how accurately the location of ion impingement may be measured, and thus insufficient spread in ion flight paths may tax the sensitivity of the detector, resulting in “coarse” positional data. To illustrate some of these shortcomings, referring to FIG. 1, the detector 100 is spaced from the local electrode 102 at such a distance that acceptable time-of-flight readings can be made (i.e., so that the desired degree of mass resolution is obtained). Since there are practical limitations on the size of the detector 100 (with larger ones generally being on the order of about 100 mm in diameter as of the year 2003), the detector 100 is generally sized such that it only rests within a portion B of the cone of evaporated ions flying from the specimen 10 (this cone of ions being designated by the reference character A). Since the detector 100 essentially captures a projection of the specimen 10, the portion of the flight cone A intersecting the detector 100 (i.e., flight cone B) defines the field of view captured by the detector 100: as the detector 100 receives more of the flight cone A, the detector 100 will image a greater amount of the specimen 10. If the detector 100 is then positioned at 100A, more distantly from the local electrode 102, it intersects even less of the flight cone A—it receives flight cone C, a subset of the ions of flight cone A—and the field of view is decreased. However, situating the detector at 100A will yield greater magnification, since the ions, when reaching 100A, have greater spread. Additionally, there is some gain in mass resolution (and thus ion identification) owing to the slightly longer time of flight. The more distant detector 100A will also provide a greater depth of analysis within the specimen 10, assuming that a set number of ions will be collected (since detection of some set number of atoms, e.g., 106 atoms, from a smaller area on the specimen 10 necessarily requires collection more deeply within the specimen 10 if the requested number of atoms are to be obtained). The various tradeoffs involved with varying the spacing between the detector 100 and specimen 10 may be summarized as follows:
SampledMassDistanceFOVMagnificationDepthResolutionIncreasesDecreasesIncreasesIncreasesIncreasesDecreasesIncreasesDecreasesDecreasesDecreases
The end result of the foregoing problems is that an experimenter may undergo the time-consuming steps of specimen preparation, atom probe warm-up, and data collection only to find that the data collected has little value: the desired feature is not within the field of view, or has insufficient magnification, or is not sampled to the desired depth, etc. This is particularly problematic where the specimen(s) are rare, expensive, or one-of-a-kind: there may not be a second chance to obtain the desired data.
In view of the foregoing issues, it would be useful to have methods and arrangements available which more readily allow the accurate collection of desired data from atom probe specimens with little or no increase in the burdens of specimen preparation, probe warm-up, and data collection/analysis.